1. Field of the Invention
The invention relates to an anti-theft system for a motor vehicle. In particular, it pertains to a locking system through which an immobilizer of the motor vehicle is released
An anti-theft system which is known from German Patent DE 44 30 360 C1 has a portable transponder that carries code information. An antenna disposed in the motor vehicle is part of an oscillating circuit that is amplitude-modulated by an oscillating circuit in the transponder If no modulated oscillation is detected on the vehicle side, then the oscillating circuit is mistuned, so that the next time an attempt is made, a modulated oscillation will be detected That can be carried out by varying the exciter frequency or the resonant frequency of the oscillating circuit.
2. Summary of the Invention
It is accordingly an object of the invention to provide an anti-theft system for a motor vehicle, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and in which a frequency-modulated transmitted signal of a transponder is detected as well as possible.
With the foregoing and other objects in view there is provided, in accordance with the invention, an anti-theft system for a motor vehicle, comprising a transceiver to be stationarily disposed in a motor vehicle, the transceiver having an antenna oscillating circuit with components including an antenna, the antenna oscillating circuit having a resonant frequency determined by the components; an oscillator imposing an oscillation on the antenna oscillating circuit for transmitting energy signals; a portable transponder having a transponder oscillating circuit oscillating at two different resonant frequencies as a function of code information, the portable transponder having an energy reservoir charged by the energy signals for transmitting the code information back to the transceiver with the aid of the oscillation of the transponder oscillating circuit; an evaluation unit receiving the oscillation generated by the transponder and received by the transceiver, the evaluation unit detecting the received oscillation and outputting the code information; a comparator demodulating and comparing the code information from the evaluation unit with desired code information and issuing a release signal for a security unit if the code information and the desired code information correspond; and the resonant frequency of the antenna oscillating circuit being adapted as a function of the transmission of the energy signals or of the reception of the oscillation to a frequency between the two resonant frequencies of the transponder oscillating circuit.
Accordingly, a transmitter in the vehicle has an oscillating circuit, which is inductively coupled with an oscillating circuit of a portable transponder in a key. In the transmitters a magnetic field or an energy signal is generated and is transmitted to the transponder where it charges the energy reservoir. By discharging the energy reservoir, a frequency-modulated transmitted signal is generated in the transponder and it is transmitted by transformer to the vehicle oscillating circuit. In order to ensure that the transmitted signal or oscillation will be reliably detected, the resonant frequency of the vehicle oscillating circuit is adapted to the resonant frequency of the transponder oscillating circuit, with the aid of an amplifier circuit.
In accordance with another feature of the invention, the antenna of the antenna oscillating circuit is a transmitter coil, the antenna oscillating circuit has capacitor connected parallel to the transmitter coil, and there is provided a circuit configuration, and an impedance connected in weighted fashion through the circuit configuration to the antenna oscillating circuit.
In accordance with a further feature of the invention, the circuit configuration is an amplifier having a voltage gain connecting the impedance in weighted fashion.
In accordance with an added feature of the invention, the circuit configuration is connected parallel to the impedance.
In accordance with a concomitant feature of the invention, the impedance has a series and/or parallel circuit of a capacitor and/or a coil and/or a resistor.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an anti-theft system for a motor vehicle, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.